Sun, Apr 12, 2020 page9

• Astronomers spot ‘missing link’ black hole — not too big and not too small 中型黑洞大發現 補足天文學關鍵環節

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  Scientists have detected a mid-size black hole — considered the “missing link” in the understanding of these celestial brutes — eviscerating an unfortunate star that strayed too close. Using data from the Hubble Space Telescope and two X-ray observatories, the researchers determined that this black hole is more than 50,000 times the mass of our sun and located 740 million light years from Earth in a dwarf galaxy, one containing far fewer stars than our Milky Way. Black holes are extraordinarily dense objects possessing gravitational pulls so powerful that not even light can escape.

  This is one of the few “intermediate-mass” black holes ever identified, being far smaller than the supermassive black holes that reside at the center of large galaxies but far larger than so-called stellar-mass black holes formed by the collapse of massive individual stars. “We confirmed that an object that we discovered originally back in 2010 is indeed an intermediate-mass black hole that ripped apart and swallowed a passing star,” said University of Toulouse astrophysicist Natalie Webb, a co-author of the study published this week in Astrophysical Journal Letters. The star was probably roughly a third the mass of the sun, Webb said.

  Webb said scientists have searched for intermediate-mass black holes for four decades and fewer than 10 good examples are known, though large numbers may exist. “So finding a new one is very significant. Also, a black hole swallowing a star happens on average only once every 10,000 years or so in any particular galaxy so these are rare occurrences,” Webb added.

  

  A cosmic homicide in action, with a wayward star being shredded by the intense gravitational pull of a black hole that contains tens of thousands of solar masses in an artist’s impression released on April 2. 四月二日發布的這張模擬圖呈現一場宇宙謀殺事件的現在進行式，一顆步入歧途的恆星，被太陽數萬倍質量的黑洞以強烈重力撕碎。

  Photo: Reuters
  照片：路透

  The supermassive black hole at the center of the Milky Way is 4 million times the mass of the sun and located 26,000 light years from Earth. The closest stellar-mass black star is about 6,000 light years from Earth. A light year is the distance light travels in a year, 9.5 trillion kilometers. Webb called intermediate-mass black holes the “missing link” in understanding the range of black holes.

  Scientists know how stellar-mass black holes — roughly three to 100 times the mass of our sun — form. They do not know how intermediate-mass black holes form but suspect that supermassive black holes arise from their mid-size brethren. “Without finding such objects, it was impossible to validate this theory,” Webb said.

  Intermediate-mass black holes have remained elusive. “The best explanation is that they are mostly in an environment that is devoid of gas, leaving the black holes with no material to consume and thus little radiation to emit — which in turn makes them extremely difficult to spot,” said University of New Hampshire astronomer and study lead author Lin Dacheng.

  

  A star being torn apart by an intermediate-mass black hole, surrounded by an accretion disc is seen in an artist’s impression released on April 2. 四月二日發布的另一張模擬圖，顯示一顆恆星遭到一個中等質量黑洞撕裂，黑洞周圍可見環繞的吸積盤。

  Photo: Reuters
  照片：路透

  (Reuters)

  中型黑洞被認為是了解這類天體暴力分子所需的「關鍵環節」，科學家近日觀測到一個中型黑洞，正在撕裂一顆遊蕩到太靠近黑洞的不幸恆星。研究人員藉由哈伯太空望遠鏡和兩座X射線天文台提供的資料，判定這個黑洞質量超過太陽五萬倍，距離地球七億四千萬光年，座落在一個恆星數量遠少於我們銀河系的矮星系中。黑洞是極高密度的星體，其引力強到連光線都無法逃脫。

  目前為止，科學家辨識出的「中等質量」黑洞為數不多，這次發現的黑洞就是其中一例，它們遠小於位在大型星系中心的超大質量黑洞，卻遠大於大型恆星坍塌後形成的「恆星質量黑洞」。法國土魯斯大學的天體物理學家娜塔莉‧韋伯指出：「我們最初在二○一○年發現的一顆天體，現在可以證實它確實是一個中等質量黑洞。它撕裂並吞噬了一顆從旁邊經過的恆星。」該研究本週發表於《天文物理期刊通訊》，韋伯為共同作者之一。她表示，被撕裂的那顆恆星質量約為太陽的三分之一。

  韋伯指出，科學家四十年來苦尋中等質量黑洞，然而目前所知足以作為範例的卻不到十個。儘管如此，這類黑洞仍可能大量存在於宇宙間。她補充說：「因此，發現一個新的中等質量黑洞具有相當大的意義。另外，在任何一個星系裡面，黑洞吞噬恆星的現象平均一萬年才會發生一次，所以這也是很稀有的事件。」

  位於銀河系中心的超大質量黑洞，其質量是太陽的四百萬倍，距離地球兩萬六千光年，最靠近地球的恆星質量黑洞距離約六千光年。「光年」指的是光（在宇宙真空中）運行一年的距離，約為九點五兆公里。韋伯認為，中等質量黑洞是了解各種黑洞類型的「關鍵環節」。

  科學家已知恆星質量黑洞如何形成，其質量約為太陽的三到一百倍不等。中等質量黑洞形成的原因至今仍然不明，但是科學家懷疑超大質量黑洞就是源自於這類中型黑洞。韋伯表示：「如果沒有發現這類天體，科學家就沒有辦法證實這項理論。」

  中等質量黑洞目前仍然難以掌握。這份研究的主要作者、新罕布夏大學的天文學家林大成指出：「最佳的解釋是，中等質量黑洞大多存在於缺乏氣體的環境中，使得黑洞沒有物質可以吞噬，導致散發出來的輻射很少──也因此讓它們極度難以發現。」

  （台北時報章厚明譯）
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